U.S. Pat. No. 6,346,188 (Shuster) assigned to ENSER Corporation and issued Feb. 12, 2002 discloses a method for cathodic protection of marine piles in which an anode is located within a jacket surrounding the pile at water level and a battery is mounted on the pile above the water level for providing an impressed current between the anode of the jacket and the steel of the pile. The anode is preferably formed of titanium or other non-corroding materials which are high on the Noble scale. However the patent mentions that other materials such as zinc can be used but these are disadvantageous since they tend to corrode. The intention is that the battery have a long life and be maintained effectively so that the impressed current remains in place during the life of the marine pile bearing in mind that the salt water in the marine environment is particularly corrosive.
Such impressed current systems can use other types of power supply including common rectifiers which rectify an AC voltage from a suitable source into a required DC voltage for the impressed current between the anode and the steel. It is also known to provide solar panels for charging batteries to be used in a system of this type.
In all cases such impressed current systems require regular maintenance and checking of the status of the power supply to ensure that the power supply does not fail leading to unexpected and unacceptable corrosion of the steel within the structure to be protected. While such maintenance can be carried out and the power supply thus ensured, this is a relatively expensive process.
Alternatively galvanic systems can be used which avoid necessity for any power supply since the voltage between the steel and the anode is provided by selecting a suitable material for the anode which is sufficiently electro-negative to ensure that a current is generated to provide a cathodic protection. These systems have obtained considerable success and are widely used.
There are two primary limitations of ordinary galvanic anodes as used in steel reinforced concrete. The first relates to the mass of zinc per anode which, depending on the required current output, limits the useful life of the anode. The second is the actual current output of the anode which may or may not be sufficient to halt corrosion of the steel. The current output is limited by the driving voltage which is essentially a fixed property, and varies with exposure conditions, age of the anode and build up of corrosion products over time.